earthrise-programming.md

Earthrise Programming

This doc provides an overview and reference for Earthrise graphics programming. Take a look at Isle 2D Drawing for blog posts on this topic. For the hardware module, see earthrise (Verilog module reference).

Overview

Earthrise instructions are 16 bits long and have one of two formats:

• 4-bit opcode and 12-bit value
• 4-bit opcode, a 4-bit function, and 8-bit options or value

Earthrise opcodes:

• 0x0 - 0x9 - load immediate into coordinate register
• 0xA - 0xB - reserved for future features
• 0xC - load immediate into colour register and control instructions
• 0xD - drawing
• 0xE - 0xF - reserved for future features

A coordinate example 0x2040:

• The 4-bit opcode is 0x2 - load immediate into coordinate register x1
• The 12-bit value is 0x040 - value is 0x40 or 64 in decimal

Breaking down a drawing example 0xD301:

• The 4-bit opcode is 0xD - draw command
• The 4-bit function is 0x3 - triangle
• The 8-bit binary options are 00000001 - a filled triangle in colour A

The triangle's coordinates and colour come from the registers. The general principle for drawing is to set coordinates and colour registers, then issues a drawing instructions.

You can generate Earthrise instructions through the Isle system library (currently being worked on) or by using the provided Earthrise assembler erasm, which converts mnemonics, such as y1 24 to underlying Earthrise instructions.

For example, using mnemonics, we can draw a line between (8, 16) and (16, 24) in line colour A (which we set to 10):

lca 10  # set line colour A to 10
x0   8  # set coordinate x0 to 8
y0  16
x1  16
y1  24
draw line ca  # draw line between (x0, y0) and (x1, y1) in line colour A

Coordinates

Earthrise uses coordinate registers to define shapes.

Coordinate registers are signed and 12 bits long, covering -2048 to +2047. There are four pairs of coordinate registers (x0, y0) to (x3, y3) for defining shapes, and one pair of translation registers. For example, when you tell Earthrise to draw a line, it starts at (x0, y0) and ends at (x1, y1).

The load immediate opcode for each register:

• 0x0, 0x1 - (x0, y0)
• 0x2, 0x3 - (x1, y1)
• 0x4, 0x5 - (x2, y2)
• 0x6, 0x7 - (x3, y3)
• 0x8, 0x9 - (xt, yt) - translation registers

The translation registers (xt, yt) are added to any future coordinates you set. For example, when (xt=4, yt=8), all following coordinates will be shifted right 4 pixels and down 8 pixels. You can use negative translation values, so (xt=-4, yt=0) will move the x coordinates left by 4 pixels. If you translate beyond the coordinate range (-2048 to +2047), the coordinates will overflow, and large positive numbers will become negative, and vice versa. Translation applies to all subsequent coordinates, so you can define a complex shape, then update the translation registers to move the whole shape without having to revise hundreds of coordinates.

Taking the line example from the overview (above), we can see how registers are set:

0x # lca 10
0x0008  # x0   8
0x1010  # y0  16
0x2010  # x1  16
0x3018  # y1  24
0xD100  # draw line ca

NB. The Earthrise command list has a 32-bit bus, so instructions are paired up in compiled .mem files, e.g. 0x0008C00A.

To express negative values in hex use two's complement: -1 is 0xFFF, and -16 is 0xFF0. For example, instruction 0x8FFC sets xt to -4, which shifts coordinates left 4 pixels.

A few things of note:

• There is no way for the CPU to read a value from an Earthrise register
• When drawing circles (discussed below), the radius r0 uses the same register and opcode as x1
• All zeros is a valid instruction: load 0 into register x0

Future Features:

• 16-bit coordinate registers with 4 bits for sub-pixel (fractional) positioning (Q12.4)
• Use of coordinate registers as blitter addresses

Colour Registers

Earthrise uses colour registers to define the line a fill colour for drawing.

Colour registers are 8 bits in length, supporting 2-256 colours. There are two colour registers for lines and two for fills; drawing instructions (discussed below) select colour A or B. A colour load immediate instruction consists of 0xC (for colour) followed by a 4-bit register ID and an 8-bit immediate.

The load immediate opcode for each register:

• 0xC0 - line colour A (lca)
• 0xC1 - line colour B (lcb)
• 0xC2 - fill colour A (fca)
• 0xC3 - fill colour B (fca)
• 0xC4 - line pattern A (lpa) - future feature
• 0xC5 - line pattern B (lpb) - future feature

The colour you see on screen is looked up in the clut by the display controller.

During Earthrise hardware initialisation, colour registers are set to 1 (one); this avoids users being confused by invisible drawings: accidentally drawing in colour 0 on a background of 0.

Drawing

Drawing instructions use the coordinate and colour registers to draw pixels, lines, and shapes.

A drawing instruction has the 0xD opcode followed by the function representing a 4-bit shape ID, and up to 8 bits of options.

• pixel (0xD0)
  • draw a single pixel at (x0, y0)
• line (0xD1)
  • draw a line from (x0, y0) to (x1, y1)
  • automatically detects horizontal lines for faster performance
• circle (0xD2)
  • draw a circle with centre (x0, y0) and radius r0 (same reg as x1)
  • does nothing if radius is zero or negative
• triangle (0xD3)
  • draw a triangle with three vertices (x0, y0), (x1, y1), (x2, y2)
  • triangles where all three vertices are the same render as a single point
• rect (0xD4)
  • draw a rect with opposite corners (x0, y0) and (x1, y1)

Drawing instructions leave the coordinate and colour registers untouched.

Drawing Options

Use up to 8 bits in the instruction to control drawing.

• Bit 0 - Filled
  • (0=outline, 1=fill) - line and pixel use fill colour if this option is used, but draw as for outline
• Bit 1 - Colour
  • (0=A, 1=B) - selects line or fill colour as appropriate
• Bit 2 - Pattern (future feature)
  • (0=A, 1=B) - select line pattern (no impact on fills)
• Bit 3 - Edge Style (0=complete, 1=connect) - future feature
  • Connect has missing edges so one shape is responsible for each line
  • Top-left rule for filled triangles/rects with connect set
  • Does not apply to pixels, lines, or circles
• Bit 4 - Draw Wrap Horizontal - shape continues beyond horizontal edge of canvas
• Bit 5 - Draw Wrap Vertical - shape continues beyond vertical edge of canvas
• Bit 6 - reserved for future use
• Bit 7 - Shape Option - extra shape-specific option - future feature
  • circle - (0=circle, 1=arc/sector) - arc/sector uses angle registers

Earthrise ensures that outline and filled shapes are aligned; an outline triangle will precisely match the edges of a filled triangle with the same coordinates.

You need two instructions to draw a shape with outline and fill, but you get to reuse the coordinates. For example, drawing an outline triangle takes 7 instructions (including 6 coordinate instructions), while an outlined and filled triangle takes one more instruction (8 in total). The draw outline instruction must follow the draw filled instruction; otherwise, the fill will cover the outline.

Control Instructions

Control instructions share opcode with colour registers, but use letters A-F for the second nibble.

• nop (0xCC00) - no operation; Continue
• stop (0xCE00) - CEase

Add a stop instruction at the end of your Earthrise drawing, otherwise Earthrise will keep executing instructions until it gets to the end of its command list.

Future Features:

• Jump and return instructions for limited flow control based on a frame counter

Assembler Mnemonics

The Earthrise assembler erasm converts instruction mnemonics into instructions. The assembler is written in Python for running on an external computer.

For coordinate registers, specify the register name and the signed immediate to load in decimal or hexadecimal. Valid immediates are in the range -2048 to 2047. Possible registers: x0, y0, x1, y1, x2, y2, x3, y3, xt, yt.

• x0 21
• y0 -1
• xt 671

Colour registers work in a similar way, but immediates are unsigned in the range 0-255. Possible registers: lca, lcb, fca, fcb.

• lca 5
• fcb 0xA

Drawing (colour):

• draw pix cb - pixel (lcb - line colour B)
• draw line ca - line (lca)
• draw circ cb - outline circle (lcb)
• draw circf cb - filled circle (fcb - fill colour B)
• draw tri ca - outline triangle (lca)
• draw trif ca - filled triangle (fca)
• draw rect cb - outline rect (lcb)
• draw rectf cb - filled rect (fcb)

The assembler uses # to begin comments.

Earthrise Drawing Examples

The following examples demonstrate how to draw basic shapes. You can see the full example in res/drawings/doc-examples.eas.

Colours

Set all the line and fill colours as you like. For these examples, I use:

lca 0xA  // line colour A = 0xA
lcb 0x8  // line colour B = 0x8
fca 0x4  // fill colour A = 0x4
fcb 0x5  // fill colour B = 0x5

Pixel

Draw a pixel at (8, 4):

x0 8         // x0 =   8
y0 4         // y0 =   4
draw pix ca  // draw pixel (line colour A)

Line

Draw a line between (8, 16) and (16, 24):

x0  8
y0 16
x1 16
y1 24
draw line ca

Draw a line between (8, 16) and (16, 24) and (8, 32):

# lines
x0  8
y0 16
x1 16
y1 24
draw line ca

# reuse existing values of x0, x1, and y1
y0 32
draw line cb

Notice how we reuse coordinate registers, saving execution time and memory.

Circle

Draw a filled circle at (160, 88) with radius 64:

x0 168
y0  96
r0  64
draw circf ca

Draw a filled circle with an outline at (160, 88) with radius 64:

x0 168
y0  96
r0  64
draw circf ca
draw circ ca

You need to draw the outline after the filled version, otherwise the fill will cover the outline.

Triangle

Draw an outline and filled triangle with vertices (60, 20), (280, 80), and (160, 164) in B colours:

x0  60
y0  20
x1 280
y1  80
x2 160
y2 164
draw trif cb
draw tri cb

Rect

Draw a filled rect with outline between (16,64) and (44,92):

x0 16
y0 64
x1 44
y1 92
draw rectf ca
draw rect ca

Quad

There isn't a quad instruction (at least for now). You can create an outline quad with four lines and a filled quad with two triangles.

Here's a filled diamond. We keep the common coordinates unchanged, which minimises the number of instructions required.

# filled quad from two triangles
x0  16
y0 128
x1  16
y1 144
x2   8
y2 136
draw trif ca
x2  24
y2 136
draw trif ca

In this case, y2 was the same for both triangles because the diamond shape is symmetrical, but this isn't generally true.

Stopping

I recommend adding a stop instruction at the end of your Earthrise instructions. Otherwise, Earthrise keeps executing until it reaches the end of the command list.